1. Field of the Invention
The present invention relates to a signal processing apparatus having a function of reproducing multiple-channel audio signals.
2. Description of the Related Art
Recently, multiple-channel audio signals represented by an audio codec such as Dolby AC-3 or DTS system are now handled by a reproduction apparatus such as a DVD (e.g., DVD-Video or DVD-Audio) apparatus. Reproduction of multiple-channel audio signals generally uses a plurality of speakers provided in front of or behind the listener. (One speaker is used for a signal of each channel.)
For example, FIG. 30 shows an exemplary arrangement of speakers for reproducing 5.1-channel audio signals in the case of the Dolby AC-3 or DTS system. As shown in FIG. 30, six speakers 5a through 5f are required.
In actuality, however, not all listeners can necessarily use six speakers (including amplifiers for driving the speakers) due to available space in their houses. Since conventional audio apparatuses such as CD apparatuses usually operate on a two-channel signal systems (left and right channels), most of the listeners are considered to be able to use two speakers. However, when multiple-channel signals are reproduced with two speakers, desired sound field effects are not obtained.
For example, it is possible that a listener who wants to enjoy sound from a DVD late at night cannot reproduce the sound at a high volume, considering that a high volume of sound will disturb the neighbors. This problem can be solved by using headphones, but desired sound field effects cannot be obtained since multiple-channel audio signals need to be reproduced using the two channels (left and right) of the headphones. There is another problem of the acoustic image being localized in the listener's head, which is specific to the headphones.
In order to solve these problems, various signal processing apparatuses for reproducing multiple-channel audio signals of, for example, the Dolby AC-3 and DTS systems using two speakers have been conceived and proposed.
FIG. 29 shows a conventional signal processing apparatus described in Japanese Laid-Open Publication No. 11-55799.
Hereinafter the conventional signal processing apparatus will be described with reference to the figures.
FIG. 29 is a block diagram of the conventional signal processing apparatus described in Japanese Laid-Open Publication No. 11-55799.
Referring to FIG. 29, reference numeral 2 represents a DVD player as a sound source, and reference numeral 3 represents a decoder for decoding a bit stream signal from the DVD player 2. Reference numerals 5a and 5b represent speakers for reproducing audio signals processed by sound image localization control through an amplifier (not shown). Reference numeral 6 represents headphones for reproducing audio signals processed by sound image localization control through an amplifier (not shown). Reference numeral 25a represents a first digital processing circuit, reference numeral 25b represents a second digital processing circuit, reference numerals 26a through 26p represent FIR filters, and reference numerals 27a through and 27d represent adders.
An operation of the signal processing apparatus shown in FIG. 29 will be described below.
A bit stream signal from the DVD player 2 is decoded by the decoder 3 into a woofer signal, a center signal, a front R signal, front L signal, a surround R signal, and a surround L signal, which are then input to the first digital processing circuit 25a. The first digital processing circuit 25a performs sound image localization control of each signal via the FIR filters 26a through 26l. Here, it is controlled so that the sound reproduced using the speakers 5a and 5b sounds as if it was reproduced using six speakers 5a through 5f shown in FIG. 30.
As an example, the case where sound from the center speaker 5c (shown in FIG. 30) is reproduced will be described. Where the transfer function of the FIR filter 26a is X1 and the transfer function of the FIR filter 26d is X2, expression (1) is formed.CR=SrrX1+SlrX2CL=SrlX1+SllX2  (1)
By finding X1 and X2 which fulfill the simultaneous equations in expression (1), the sound from the center speaker 5c (the speaker indicated by the dashed line in FIG. 29) can be reproduced using speakers 5a and 5b. 
Namely, the transfer functions X1 and X2 of the FIR filters 26c and 26d can be found by expression (2).X1=(SllCR−SlrCL)/(SrrSll−SrlSlr)X2=(SrrCL−SrlCR)/(SrrSll−SrlSlr)  (2)
By performing the same processing for the signals of the other channels, it is controlled so that the sound reproduced using the speakers 5a and 5b sounds as if it was reproduced using six speakers 5a through 5f shown in FIG. 30.
Then, the output from the first digital signal processing circuit 25a is input to the second digital signal processing circuit 25b. Thus, sound image localization control is performed for the case of using the headphones 6. It is controlled so that the sound reproduced by the headphones 6 sounds as if it was reproduced using the speakers 5a and 5b. 
Where the transfer function of the FIR filter 26m is Y1, the transfer function of the FIR filter 26n is Y2, the transfer function of the FIR filter 26o is Y3, and the transfer function of the FIR filter 26p is Y4, expression (3) is formed.Srr=HrrY1Srl=HllY2Slr=HrrY3Sll=HllY4  (3)
In expression (3), Hrr is the transfer function from the right speaker of the headphones 6 to the right ear of the listener, and Hll is the transfer function from the left speaker of the headphones 6 to the left ear of the listener. By finding Y1, Y2, Y3 and Y4 which fulfill the equations of expression (3), the sound from the speakers 5a and 5b can be reproduced using the headphones 6.
Namely, the transfer functions Y1, Y2, Y3 and Y4 of the FIR filters 26m through 26p can be found by expression (4).Y1=Srr/HrrY2=Srl/HllY3=Slr/HrrY4=Sll/Hll  (4)
Hereinafter, another conventional signal processing apparatus will be described.
FIG. 31 is a block diagram of another conventional signal processing apparatus.
Referring to FIG. 31, reference numeral 2 represents a DVD player as a sound source, and reference numeral 3 represents a decoder for decoding a bit stream signal from the DVD player 2. Reference numeral 4 represents a DSP for performing sound image localization control. Reference numerals 5a and 5b represent speakers for reproducing audio signals processed by sound image localization control performed by the DSP 4 through an amplifier (not shown). Reference numeral 6 represents headphones for reproducing audio signals processed by sound image localization control performed by the DSP 4 through an amplifier (not shown). Reference numeral 7 represents a transfer function correction circuit implemented by a program executed by the DSP 4. Reference numerals 9a through 9l represent FIR filters included in the transfer function correction circuit 7. Reference numerals 11a and 11b represent adders implemented by a program executed by the DSP 4. Reference numerals 12a and 12b represent subtractors implemented by a program executed by the DSP 4. Reference numerals 13a and 13b represent crosstalk cancel circuits implemented by software of the DSP 4.
An operation of the signal processing apparatus shown in FIG. 31 will be described below.
A bit stream signal from the DVD player 2 is decoded by the decoder 3 into a woofer signal, a center signal, a front R signal, front L signal, a surround R signal, and a surround L signal, which are then input to the DSP 4. The DSP 4 performs sound image localization control of each signal by the transfer function correction circuit 7. The output signal from the transfer function correction circuit 7 is divided into two channels by the adders 11a and 11b and then output to the headphones 6 or the speakers 5a and 5b. When the speakers 5a and 5b are used, the crosstalk cancel circuits 13a and 13b and the subtractors 12a and 12b act to remove the influence of crosstalk transfer functions Srl and Slr from the speakers 5a and 5b to the left and right ears of the listener.
The transfer function correction circuit 7 performs sound image localization control of the signal of each channel in the case when the speakers 5a and 5b or the headphones 6 is used. Specifically, the signal of each channel is convoluted with the coefficient which represents each transfer function by each of the FIR filters 9a through 9l. 
As an example, the case where sound from the center speaker 5c (shown in FIG. 30) is reproduced using the speakers 5a and 5b will be described. In the following description, the transfer function of the FIR filter 9c is X1 and the transfer function of the FIR filter 9d is X2.
The crosstalk cancel circuits 13a and 13b act as follows. The output from the crosstalk cancel circuits 13b is subtracted from the output from the adder 11a, and thus the crosstalk transfer function Srl from the right speaker 5a to the left ear of the listener is counteracted. The output from the crosstalk cancel circuits 13a is subtracted from the output from the adder 11b, and thus the crosstalk transfer function Slr from the left speaker 5b to the right ear of the listener is counteracted. Due to such an action of the crosstalk cancel circuits 13a and 13b, expression (5) is formed.Transfer function of crosstalk cancel circuit 13a=Srl/SllTransfer function of crosstalk cancel circuit 13b=Slr/Srr  expression (5)CR=Srr{X1−(Slr/Srr)X2}+Slr{X2−(Srl/Sll)X1}CL=Srl{X1−(Slr/Srr)X2}+Sll{X2−(Srl/Sll)X1}  expression (6)
By finding X1 and X2 which fulfill expression (6), the sound from the center speaker 5c (the speaker indicated by the dashed line in FIG. 31) can be reproduced using speakers 5a and 5b. 
Namely, the transfer functions X1 and X2 of the FIR filters 9c and 9d can be found by expression (7).X1=SllCR/(SrrSll−SlrSlr)X2=SrrCL/(SrrSll−SrlSlr)  (7)
By performing the same processing for the signals of the other channels, it is controlled so that the sound reproduced using the speakers 5a and 5b sounds as if it was reproduced using six speakers 5a through 5f shown in FIG. 30.
Hereinafter, the case where the sound from the center speaker 5c is reproduced using the headphones 6 will be described.
Where the transfer function of the FIR filter 9c is X1 and the transfer function of the FIR filter 9d is X2, expression (8) is formed.CR=HrrX1CL=HllX2  (8)
In expression (8), Hrr is the transfer function from the right speaker of the headphones 6 to the right ear of the listener, and Hll is the transfer function from the left speaker of the headphones 6 to the left ear of the listener. By finding X1 and X2 which fulfill the equations of expression (8), the sound from the speaker 5c can be reproduced using the headphones 6.
Namely, the transfer functions X1 and X2 of the FIR filters 9c and 9d can be found by expression (9).X1=CR/HrrX2=CL/Hll  (9)
By performing the same processing for the signals of the other channels, it is controlled so that the sound reproduced using the headphones 6 sounds as if it was reproduced using six speakers 5a through 5f shown in FIG. 30.
As can be appreciated from the above description, in this conventional example, the coefficients of the FIR filters 9a through 9l need to be changed in the case where speakers 5a and 5b are used from in the case where the headphones 6 are used.
In this conventional example, it is intended that the transfer function including reflection is realized by the FIR filters 9a through 9l. Therefore, the number of taps of each of the FIR filters 9a through 9l needs to be sufficient to fully simulate the impulse response of the room to be mimicked. FIGS. 32 and 33 show the coefficients when the number of taps is 1024. (In FIG. 31, the number (1024) provided regarding the FIR filters 9a through 9l represent the number of taps.) FIG. 33 shows the coefficients by expanding the curve in FIG. 32 in the direction of the level so that the reflection component is more clearly shown. Since the sampling frequency is 48 kHz, the time length of 1024 taps is about 21 msec. This is converted into a distance of about 6 m. This approximately corresponds to a 8-“tatami mat” listening room, in which the primary reflection is barely accommodated. Higher-order reflection such as a reverberation component cannot be reproduced at all. In a larger room, even the primary reflection is not accommodated, and a larger number of taps are necessary. In accordance with this, the calculation amount and the memory capacity are increased.
Hereinafter, still another conventional signal processing apparatus will be described.
FIG. 34 is a block diagram of still another conventional signal processing apparatus.
Referring to FIG. 34, reference numeral 2 represents a DVD player as a sound source, and reference numeral 3 represents a decoder for decoding a bit stream signal from the DVD player 2. Reference numeral 4 represents a DSP for performing sound image localization control. Reference numerals 5a and 5b represent speakers for reproducing audio signals processed by sound image localization control performed by the DSP 4 through an amplifier (not shown). Reference numeral 6 represents headphones for reproducing audio signals processed by sound image localization control performed by the DSP 4 through an amplifier (not shown). Reference numeral 7 represents a transfer function correction circuit implemented by a program executed by the DSP 4. Reference numeral 8 represents a reflection circuit implemented by a program executed by the DSP 4. Reference numerals 9a through 9l represent FIR filters included in the transfer function correction circuit 7. Reference numerals 10a through 10l represent delay lines included in the reflection circuit 8. Reference numerals 11a and 11b represent adders implemented by a program executed by the DSP 4. Reference numerals 12a and 12b represent subtractors implemented by a program executed by the DSP 4. Reference numerals 13a and 13b represent crosstalk cancel circuits implemented by software of the DSP 4.
The signal processing apparatus shown in FIG. 34 includes the reflection circuit 8 connected in series to the transfer function correction circuit 7, in addition to the structure of the signal processing apparatus shown in FIG. 31. The number of taps of each of the FIR filters 9a through 9l included in the transfer function correction circuit 7 is smaller than that of the signal processing apparatus shown in FIG. 31 (i.e., 128 taps). That is, the transfer function correction circuit 7 and the reflection circuit 8 in FIG. 34 are intended to realize a transfer function which is equivalent to the transfer function of the transfer function correction circuit 7 shown in FIG. 31.
FIG. 35 shows an internal structure of each of the delay lines 10a through 10l included in the reflection circuit 8.
Referring to FIG. 35, reference numerals 14a through 14N represent N number of delay devices, reference numerals 15a through 15N represent N number of level adjusters, reference numerals 16a through 16N represent N number of frequency characteristic adjustment devices, and reference numerals 17a through 17N represent N number of adders.
A signal input to each of the delay lines 10a through 10l is output through the adders 17a through 17N without being processed. The signal is also processed as follows. The signal is provided with a predetermined delay time by each of the delay devices 14a through 14N, and the outputs from the delay devices 14a through 14N are level-adjusted by the respective level adjusters 15a through 15N. The output from the level adjusters 15a through 15N are frequency-adjusted as predetermined by the respective frequency characteristic adjustment devices 16a through 16N. The frequency adjustment is, for example, to vary the level of a component of a certain frequency band or to perform low pass filtering. Then, the outputs from the frequency characteristic adjustment devices 16a through 16N are added, by the adders 17a through 17N, together and with the signal component which has been input to each of the delay lines 10a through 10l but which has not been processed. In other words, the delay lines 10a through 10l each add a direct sound component as an input signal (i.e., an output signal from the respective one of the FIR filters 9a through 9l) and N number of independent reflection components processed by the delay devices 14a through 14N, the level adjusters 15a through 15N, the frequency characteristic adjustment devices 16a through 16N and the adders 17a through 17N.
Accordingly, signals other than the direct sound component, i.e., components from a front portion of the impulse response (a primary reflection obtained by the floor is located at a relatively front portion) to a rear portion (reverberation component or the like) are realized by the reflection circuit 8. In other words, the reflection circuit 8 simulates the impulse response of the listening room to be mimicked. Therefore, the number of taps of each of the FIR filters 9a through 9l can be reduced. The reason for this is because the FIR filters 9a through 9l need to only reproduce the direct sound component instead of the impulse response of the entire listening room, as opposed to the case of FIG. 31 in which the FIR filters 9a through 9l need to reproduce the impulse response of the entire listening room. The measurement of the direct sound component in the case of FIG. 34 may be performed in an anechoic chamber. FIG. 36 shows the coefficients measured in an anechoic chamber when the number of taps is 128 (In FIG. 34, the number (128) provided regarding the FIR filters 9a through 9l represent the number of taps.)
The calculation time of the delay lines 10a through 10l can usually be suppressed to be shorter than the calculation time of the FIR filters, which have a large number of taps. Hence, the structure in FIG. 34 can reduce the calculation time as compared to the structure in FIG. 31.
As described above, the structure shown in FIG. 34 provides approximately the same level of sound image localization control effect as that of the structure shown in FIG. 31.
The conventional structures shown in FIGS. 29, 31 and 34, however, have the following problems.
In the conventional structures shown in FIG. 29, the first digital processing circuit 25a performs virtual sound image localization control of multiple-channel signals for the speakers 5a and 5b, and the second digital processing circuit 25b performs virtual sound image localization control of the signals reproduced by the speakers 5a and 5b for the headphones 6. Accordingly, the audio signals twice processed with virtual sound image localization control are obtained through the headphones 6. Usually, even when the virtual sound image localization control is performed once, it is difficult to perfectly reproduce the sound produced by, for example, the speakers 5a and 5b in FIG. 30 located in a certain room due to individual differences, dispersion in the speaker or headphone characteristics, processing precision errors (e.g., precision of the FIR filter coefficients) and the like. Thus, even the sound image localization of the output signal from the first digital processing circuit 25a is not as precise as desired. When the sound image localization is performed again by the second digital processing circuit 25b, the effect is further deteriorated to the level of being useless.
The conventional signal processing apparatus shown in FIG. 29 assumes only a multiple-channel signal source of six channels or 4 channels (for example, a DVD player). Structures for performing sound image localization control of a conventional stereo sound source such as a CD player are not described. Even if the structure shown in FIG. 29 is used for the stereo sound source, it is merely that the signals other than the front L signal and the front R signal are not input. Use of the calculation amount and the memory capacity which were required for the center signal and the surround signals in order to improve the processing precision of the front L signal and the front R signal is not described. The DVD Standards include PCM 2-ch mode in addition to the multiple-channel mode, in which case a similar problem occurs.
In other words, the structure shown in FIG. 29 cannot be used for effectively utilizing a limited calculation amount in accordance with the number of input channels.
In the structures shown in FIGS. 31 and 34, virtual sound image localization is performed once by the transfer function correction circuit 7. Like the structure in FIG. 29, the structures shown in FIGS. 31 and 34 are not for effectively utilizing a limited calculation amount in accordance with the number of input channels.